Rail Engineer has covered the upgrade of the so-called Core Valley Lines (CVL) extensively. In Issue 197 (July/August 2022) we reported on construction progress including aerial shots of site clearance for Taff’s Well depot. By November 2024, we were able to travel on one of the new trains and visit the new depot and its new Class 398 trains. This latest article describes some of the challenges that engineers overcame in delivering the upgrades.
In passing, it’s worth restating that the lines from Cardiff Queen Street to Cardiff Bay, Rhymney, Coryton, Treherbert, Aberdaren, and Merthyr Tydfil are under the control of Transport for Wales (TfW) and this has provided both opportunities and challenges. On completion of the South Wales Metro project, the signalling of these lines will also be transferred from Network Rail to TfW, with teams based out of the newly built Integrated Control Centre in Taff’s Well.
This is a significant project, involving 218km of track, two depots, two sets of stabling sidings, and three new fleets of diesel, bi-mode and tri-mode trains. The overhead line equipment, which will be used to electrify 170km of track, is based on the UK Master Series 100 design range. The Valley railway lines are over 150 years old, originally used to transport coal from the South Wales Valleys into Cardiff, with a lot of the structures such as bridges and viaducts incompatible with 25kV OLE gauge and clearances. It was therefore decided to use discontinuous electrification to minimise the number of civil engineering interventions and the amount of track lowering to reduce capital expenditure.
Two varieties
Discontinuous electrification comes in two varieties: permanently earthed sections (PES) – similar to long neutral sections – where the pantograph remains raised; and catenary free sections (CFS) where the pantograph has to be lowered. While the use of CFS and PES reduces the overall cost of installing OLE, avoiding costly civil engineering interventions, the complex system does require additional maintenance over its lifespan.
Conventional electrification involves a relatively straightforward interface between train and OLE requiring a robust AC supply to power the trains continuously. Discontinuous electrification adds complication. To keep the trains moving through both PES and CFS, batteries are required on the trains. This leads to the need for collaborative system modelling between the electrification and rolling stock teams. ‘How big should the battery range be?’ and ‘What extra electrical capacity needs to be provided to charge batteries?’ are just two of the questions that had to be addressed. Modelling led to a decision on battery range, a conclusion necessary to allow the trains to be designed. The infrastructure team then had to manage within this constraint.
This meant, for example, that if one area was found not to be suitable for live OLE, the models had to be re-run to identify an area elsewhere where live OLE could be extended while not exceeding the battery range/charging parameters already determined. There are also challenges arising from the capacitance of the ground-based 25kV cables required to link the live OLE sections together.
It is worth mentioning that providing batteries on the trains delivered benefits in stabling and depot areas. For example, at Taff’s Well depot the only OLE is on the straight stabling roads and then only for battery charging. Train movements are entirely by battery power. This means that complex OLE ‘knitting’ through points and crossings and in maintenance sheds is avoided. The exception is one shed road where there is a retractable rigid OLE section for testing. There is also the opportunity to avoid OLE in areas that are sensitive to, or too complex for, OLE installations. Some areas that are not wired include:
- Complex stations: Pontypridd, Cardiff Queen St.
- Grade 2 listed structures: Llanbradach & Hengoed footbridges, Merthyr Viaduct, Rhondda Viaduct, and Caerphilly Tunnel.
- Minimising EMI Interference risk: Cardiff University’s sensitive medical scanning equipment.
Design of PES and CFS
Permanently earthed sections are not new to UK railways. Examples are on the Romford-to-Upminster branch in east London and on the Paisley Canal electrification in Scotland where trains were required to coast through them, not unlike neutral sections. For the CVL, the sections are often much longer, hence the on-train battery requirement. A great deal of effort was put into the design to ensure that the sections are indeed earthed, although staff are still instructed to treat them as live unless the OLE isolation and earthing process has been applied.
The live and earthed sections are joined by insulating components. On approach to a PES, a beacon in the four-foot (see Beacons) instructs the train to switch to battery power. At the end of the PES, once 25kV is detected by the train, it switches back to that supply.
When approaching catenary free sections, it is necessary to ensure that the pantograph is down. The main command comes from a beacon instructing the train to lower the pantograph and switch to battery power. As a back-up, in the event that this message is not received, the catenary wire height will gradually increase to above the height where the pantograph’s over-height detection forces it to lower (see diagram). When the train has passed the obstruction and is, once again, under the catenary, it passes a second beacon which instructs the pantograph to be raised and to switch back from battery to 25kV operation.
All this required significant safety analysis and, in many instances, TfW working with Amey Infrastructure Wales applied their own product acceptance and assurance processes. Some of the issues faced by engineers, safety assessors, and assurers included:
- The possibility that the extensive use of ground level HV cables might lead to unexpectedly high voltages in low load conditions while considering if shunt reactors might be required.
- Whether discharge to earth from the ground based HV cables would be higher than expected.
- Managing HV cables through CFS or PES stations while maintaining under platform recess.
- Design variations made necessary by both the geology of the land and ground conditions resulting from past mining operations.
Two fleets
Two fleets will operate on the Core Valley Lines once the Metro is completed. These consist of the Class 398 bi-mode tram-train and Class 756 tri-mode train. The former was described in Issue 212 (Jan-Feb 2025). The Class 756 fleet comprises three- and four-car units, each with a central ‘power pack’. These units can be powered using the 25kv OLE, and diesel/ battery power through non-electrified sections.
The power module housed three battery modules and a 480kW diesel generator. The maximum output through the four motored axles is 2.6MW on 25kV and 1.3MW on battery. All three modes are controlled by a power converter/energy manager which sets the power source to the appropriate conditions. Regenerated braking energy is used to charge the batteries and any excess is fed back into the OLE. There is a diesel/battery mode for travelling on longer, non-electrified routes. The diesel engine takes some of the load, preserving battery charge and extending the range. At locations where the electrical load is low, such as when stopped at a station or when coasting, the engine can charge the batteries.
The Class 756 entered service in November 2024 on the Merthyr and Aberdare lines and then entered service onto the Treherbert line in February 2025. Its introduction marked the first time the beacon system was used on the CVL to allow for automatic power mode changeover.
As this was a novel system to TfW, and several parties were involved, a great deal of effort was put into building relationships to ensure any teething issues with this automatic power changeover were thoroughly investigated. To support the investigation of any emerging issues, a cross-party working group was formed, which developed a Failure Reporting, Analysis and Corrective Action System.
Due to collaborative efforts to investigate incidents, new software is being implemented to address issues. This has led to improved performance and reliability of the Class 756 trains on the Merthyr, Aberdare, and Treherbert lines. As of June 2025, 14 Class 756 trains are operational on these lines, replacing all legacy Class 150 units.
Post-completion
On completion of the transformation project, the plan is to run Class 756 trains on the Rhymney and Coryton lines. Services will run from Rhymney through Cardiff and onwards to Barry Island and Bridgend via the Vale of Glamorgan. As the section of track South of Cardiff Central through to the Vale of Glamorgan is under the ownership of Network Rail, the 756s will make use of their diesel generator to travel on this non-electrified section.
The Class 398 trains will run on the Treherbert, Aberdare, Merthyr Tydfil, and Cardiff Bay lines. The tram-trains will run only within the boundaries of the CVL, utilising 25kV from the overhead wires and battery power through short non-wired sections. Their unique ability to run as a tram on light rail infrastructure will also be utilised as part of ‘Cardiff Crossrail’, a project to install a new tram-line connection from Cardiff Bay, Cardiff Central, and potentially beyond.
In July 2025, five additional Class 756 trains will be introduced onto the Coryton and lower Rhymney line, travelling between Coryton, Caerphilly, and Penarth. The Coryton and lower Rhymney lines are now fully electrified, following the completion of infrastructure works in Spring 2025.
Daytime proving runs of the Class 398 tram-trains has begun on the Merthyr, Aberdare, and Treherbert lines. TfW can therefore begin to see the planned transformation of these services being realised. Engineers are looking forward to validating the extensive modelling that was carried out during the system design with a very large number of variables and assumptions. Other promoters of discontinuous electrification systems will be watching closely.
This article is based on a presentation given in April 2025 to the IMechE South Western Centre by Ian Edwards and Alexander Evans from Transport for Wales.
Beacons: APMC – ASDO
The system used for controlling switching to battery power and for raising/lowering the pantograph is known as Automatic Power Mode Change (APMC). It is piggy-backed onto the Automatic Selective Door Open (ASDO) system required on these trains. The system used is a development of the Sella Controls’ Tracklink III system used extensively for ASDO on other fleets and for the positive prevention of overspeeding system on Corydon’s trams. It comprises on-train readers and track mounted Radio Frequency Identification (RFID) tags. The only input required from the driver is the initial destination setting. The rest is managed though the system’s database.
Image credit: Malcolm Dobell
